At present the light source used by liquid crystal display (LCD) of desktop computers and notebook computers, PDA, and Webpad generally is a fluorescent lamp (such as cold cathode fluorescent lamp—CCFL) actuated by a high voltage. Light is projected to a backlight plate to enable users to see the displaying picture.
LCD TV or LCD display screen with a touch film requires a greater luminosity to compensate visual requirement. As the fluorescent lamp is driven by a high voltage, when the current is higher, the luminosity also is greater. Moreover, in order to increase the luminosity, a plurality of fluorescent lamps are often being used. In such an occasion, the most important issue is to maintain the evenness of lamp current among the loads or minimize the characteristic tolerance among them. Moreover, with increased number of the fluorescent lamps, the number of control units to actuate and ignite the lamps also increases. As a result, the size of the circuit board increases. Fabrication is more difficult and cost is higher. In the event that a plurality of fluorescent lamps are used to increase the luminosity and evenness, because of the tolerance of the fluorescent lamps, uneven tube current and luminosity often occur to the fluorescent lamps. It makes selection of the fluorescent lamps more difficult or results in more fluorescent lamps are needed to improve the luminosity and evenness. Hence the cost becomes even higher, and fabrication and adjustment are even more difficult. The conventional method for controlling the current of fluorescent lamps is generally as follow:
Refer to FIG. 1 for a conventional actuating device for driving and igniting fluorescent lamps. It includes a power supply 10, a high frequency pulse modulator 11, a power switch 12, a conversion unit 13, a piezoelectric transformer 14 and two fluorescent lamps 15 and 15′. The two fluorescent lamps 15 and 15 are jointly connected to a resistor 16 and are grounded. When a voltage is input, the power switch 12 actuates the piezoelectric transformer 14 through the conversion unit 13 to generate a high voltage output because of inverted/positive piezoelectric effect and actuate and ignite the fluorescent lamps 15 and 15′. Due to the piezoelectric transformer 14 generates a positive half cycle signal and a negative half cycle signal to actuate the fluorescent lamps 15 and 15′ at the same time, and the fluorescent lamps 15 and 15′ output a positive half cycle signal and a negative half cycle signal that cancel out each other by grounding, there is no voltage drop on the resistor 16 end. As a result, the output ends of the fluorescent lamps 15 and 15′ also have no voltage drop. Hence it is not possible to feed back a voltage signal to the high frequency pulse modulator 11. Therefore it is not possible to control the current of the fluorescent lamps 15 and 15′ through a voltage feedback circuit. Moreover, the conventional fluorescent lamps 15 and 15′ also have other problems, such as unstable electrodes, gas, and the like. They will cause abnormal operation of the current in the fluorescent lamps 15 and 15′ (such as power loss). The conventional technique previously discussed uses merely one feedback circuit to connect the high frequency pulse modulator 11, and cannot control the current in the fluorescent lamps 15 and 15′. This is because the fluorescent lamps 15 and 15′ have to be ignited by a high voltage to generate a high energy current. It is easy to generate high frequency noise or arc effect. This phenomenon tends to result in power loss or damages of the fluorescent lamps 15 and 15′.